The present invention relates to a method for extruding a plastic member so that the member has a surface with a fine, un-oriented roughness. The extruded member may be a carrier strip for exchangeably holding optically scannable information sheets, particularly labels with bar codes. Such a carrier strip preferably includes fastening means for fixing the carrier strip to a support such as the edge region of a shelf, container, or the like. The carrier strip has a receiving pocket formed between a rear wall and a transparent front wall. The receiving pocket accommodates the labels.
Such a holding strip is described in U.S. Pat. No. 4,295,288. In the patent, the fastening means is composed, for example, of a latch strip which may be inserted into a latch groove at the edge regions of shelves, containers, or the like. Connected in one piece with the fastening means is a covering strip for covering the edge regions of the shelves, containers, or the like as well as the rear wall and the front wall, which together form the receiving pocket for the labels. The rear wall and the front wall are connected with one another at the base, for example by way of a film hinge, and the upper edge of the rear wall grips over the free edge of the front wall in a covering manner. The front wall may be released from this retention by a slight pressure, thus causing the receiving pocket to open for the insertion of a label. Thereafter, slight pressure against the rear wall closes the receiving pocket again, with the upper edge of the rear wall snapping over the free edge of the front wall and keeping it closed.
Another carrier strip is described in European patent 203,209. This carrier strip is composed of a fastening strip and a covering strip which follows in one piece at the lower edge of the fastening strip and lies against the fastening strip. In order to insert a label or the like, this covering strip may be moved away at one side. A cover is attached to the upper edge of the fastening strip so as to project beyond the upper edge of the covering strip and serve as an arresting edge. In contrast thereto, the upper edge of the covering strip is configured as an attached latch strip which forms a snap connection with the arresting edge of the fastening strip in that it engages underneath the arresting edge. The carrier strip is made from a single piece of a polymeric material and includes a transparent covering strip which permits proper viewing and reading of the label. In contrast thereto, the fastening strip may be made of an opaque plastic and its rear face is available as a fastening surface which allows it to be glued to a shelf, for example.
The carrier strips described above are suitable for use on shelves in warehouses, storage facilities employing high shelf arrangements, and the like. The labels and the like disposed in the carrier strip ma be read by operating personnel as well as customers passing by the shelves. However, reading the labels with the scanner pen of a bar code reader is preferred in more recent shelf structures. Scanning devices such as bar code readers operate according to the reflection method, also called the direct light method. Light-emitting diodes operating primarily in the infrared region are typically employed as light sources. However, high intensity red-light diodes may also be used.
The light receiver for the bar code reader is typically a photodiode. Because the transmitting diode and the photodiode must be accommodated next to one another in a very small space, integrated systems exist which incorporate a light-emitting diode, a photodiode, a filter plate, and an amplifier for the photodiode signal. A frequently-employed system is the type HEDS-1000 sensor made by Hewlett-Packard. This sensor operates with infrared light of a wavelength of 700 nm and can be operated in a controlled or uncontrolled manner. Passage of the scanning pen of the bar code reader over a bar code on a label causes the infrared light to be received by the photodiode, with the width and the spacing of the bar codes appearing as "flickering lights" which are converted into electrical signals for electrical processing and evaluation.
Due to the high light intensity of the light-emitting diode and the fact that the photodiode is usually operated in a controlled manner in a bar code reader, no interference with operation occurs if, for example, the background of the label on which the bar code is printed fluctuates in hue within certain limits or the printing of the bar code appears in different shades of black.
Carrier strips can be made by continuously extruding plastic, such as polyvinyl chloride, through a suitably configurated die. Co-extrusion of different plastic compositions may be employed. In the extrusion process, it inevitably happens that so-called drawing scratches or dielines are produced on the surfaces of the walls of the extruded object. This includes the front wall of an extruded carrier strip. Dielines on a portion of the front wall 100 of a prior art extruded carrier strip are illustrated schematically in FIG. 5 (in which the surface irregularities have been exaggerated for illustration purposes). Dielines occur because lubricants used in the plastic composition "plate out" and form tiny lumps on the extruder die. These plated-out lumps make parallel impressions on subsequently extruded plastic. The dielines may be so slight in elevation that the human eye is barely able discern them. However, since the light-emitting diodes of scanning pens have an extremely high light intensity, the dielines on the transparent front wall of a prior art extruded carrier strip may reflect the IR light from the light-emitting diode, so that the hill-and-valley formations which together form the dielines appear to a bar code reader as additional light-to-dark transitions. Bar code readers tend to be extremely sensitive to such "false" reflected light.
In FIG. 5, rays 102 are transmitted from a scanning pen through the conventional front wall -00 to a label 104 with a bar code. However, light may also be reflected by the dielines on either the front or rear surface of wall 100, as indicated by rays 106 and 108. The dielines extend longitudinally and are parallel to one another, so in practice further rays parallel to rays 106 and 108 would be reflected in planes above and below the plane of FIG. 5. Because the dielines cause additional reflections, they give erroneous information to the bar code reader, which confuses its electronic evaluation system (not illustrated). Thus, erroneous information may be fed into the electronic evaluation system when a bar code on a label is read with a bar code scanner, thereby negating some of the value of scanning-in information.
In addition to the ingredients that are reacted and homogenized during extrusion of a plastic member the pressure and temperature of the plastic as it passes through the die play a decisive role. For example, the temperature must not be too high in order to avoid heat damage to the plastic. Furthermore the use of an inappropriate extrusion pressure may cause damage to the surface of the extruded member. It is a characteristic of the plastic extrusion process that a member formed by extruding molten plastic (also known herein as a plastic "melt") at a normal pressure has a smooth surface, albeit with dielines, but if the extrusion pressure is increased too far the surface becomes fissured. One can term this a characteristic "melt fracture" of a plastic melt that is extruded at too high a pressure. The explanation for this phenomenon may be that, with increasing pressure, elastic vibrations occur in the melt stream due to cross-sectional changes in the extrusion apparatus or due to intermittent wall adhesions, and these elastic vibrations can no longer be attenuated in the subsequent portions of the extrusions apparatus.